Projects
The Cost Effectiveness of Approach Guardrail on County State-Aid Bridges
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Description:
Rationale:
Overview:
Certain bridge components, including bridge railings, piers, headwalls, and abutments are fixed-objects that are typically very close to the edge of the traveled way. As such, their presence in the clear zone constitutes a roadside safety hazard. Guardrail and other treatments, such as attenuators, are often connected to the ends of the rail/parapet to protect vehicles from striking the less-forgiving concrete ends of the rail. FWHA requires bridge approach guardrail on all NHS roadways, but states are given discretion to develop their own policies or guidelines for non-NHS roadways, such as county state-aid roadways. The Minnesota Department of Transportation (MnDOT) currently requires guardrail to be placed on the approach to bridges on the county state-aid bridges if the average daily traffic (ADT) exceeds 750 vehicles per day, although previous MnDOT standards required bridge approach guardrail on county state-aid bridges with ADTs greater than 400. But what are the appropriate criteria for placement of guardrail on bridge approaches? The objective of this research was to determine the ADT in which the benefit/cost ratio suggests installing bridge approach guardrail provides a positive return on investment for state-aid bridges in Minnesota. In other words, identify the ADT were benefits exceed costs and the benefit/cost ratio exceeds 1.0.
There is no debating the fact that hazardous immovable objects such as bridge abutments, piers, and railings exist within roadway clear zones on state-aid roadways and provide a significant safety concern. Yet the installation of guardrail on these low volume roads may add costs and other safety and maintenance problems that outweigh the proposed benefits. Beyond the fact that guardrail itself is a fixed object within the clear zone, guardrail is know to effectuate snow drifting during the winter months providing an additional maintenance and safety concern. Additionally, grass and weeds that grow near the guardrail cannot be cut by traditional lawn mowers requiring county workers to use labor-intensive weed cutting devices around guardrail posts.
Two AASHTO publications are widely regarded as the leading guidelines for clear-zone protection. The AASHTO Roadside Design Guide (RDG) provides guidance in evaluating the need for protection of roadside objects, including bridge rail. Tables in the RDG provide minimum clear zone requirements based on design speed, ADT, and slope. For example, for a roadway with design speed of 55 mph, ADT less than 750, and fore slopes flatter than 1:4, a minimum clear zone of 12-18 feet is required. However, the RDG, does not specifically address design issues with very low-volume local roads (i.e., ADT ≤ 400), which are of specific interest to the research described here.
Perhaps a better tool for addressing design issues on very low-volume local roads is the AASHTO Guidelines for Geometric Design of Very Low-Volume Local Roads (VLVRDG) (ADT ≤ 400), which states that roadside clear zones provide very little benefit on low volume roads. Traffic barriers are not generally cost-effective on roads with very low traffic volumes because the probability of striking a fixed object on these types of roads is extremely low (compared to similar roadways with higher traffic volumes). However, the AASHTO VLVRDG applies only to roads that are functionally classified as a local road and have a design ADT of 400 vpd or less because of the high level of driver familiarity that is associated with these types of roads.
One of the initial tasks of the research was to conduct a survey of state transportation agencies to determine the state-of-practice for approach guardrail application on county state-aid bridges. Twenty-six of the 35 responding state agencies (74%) have policies or guidelines requiring the placement of guardrail or attenuation on all bridge approaches if state monies were used regardless of the roadway system – essentially mirroring the FHWA requirement for NHS roadways. Only Wisconsin, Illinois, and Virginia have policies similar to Minnesota’s requiring bridge approach guardrail on state-aid local highways only where an ADT threshold is exceeded. Six of the responding agencies indicated that lower-speed facilities (i.e., speeds ≤ 45 mph) don’t require approach guardrail.
The primary task for this research was to perform a benefit/cost analysis of approach guardrail on county state-aid bridges. To accomplish this task, the researchers obtained data for 398 Minnesota county state-aid bridges from 10 Minnesota counties. The 398 bridges were divided into two samples: those with approach guardrail (133) and those without approach guardrail (265). The presence of guardrail was confirmed by the local jurisdiction. MnDOT’s bridge database was queried to obtain relevant information about the bridges, including: reference point ID, location, year built, max span length, number of lanes, functional class, and ADT.
The Minnesota crash database was queried to obtain the crashes occurring near the 398 bridges included in the analysis. All crashes from 1988 – 2002 that occurred ±1 mile from the bridge were obtained. This sample was manually filtered down to include only crashes occurring near the bridges (i.e., within ~200 ft) that would likely be affected by the presence or absence of approach guardrail. Only single-vehicle fixed-object or run-off-the-road crashes were included in the final crash sample. The resulting crash data set included a total of 263 crashes occurring between 1988 and 2002 that met the aforementioned criteria. Of the 263 crashes, 143 crashes occurred at bridges with approach guardrail and 120 crashes occurred at bridges without approach guardrail.
Researchers are currently analyzing the crash data and performing a series of benefit/cost analyses. Researchers working with MnDOT staff have determined that installation and maintenance of approach guardrail will cost approximately $15,000 over a 30-year life cycle. Preliminary findings from the crash analysis show very little differences in crash frequency between bridges with and without approach guardrail, although some variation in frequency is found when bridges are separated by ADT category (see graph at right). A draft final report of the research results is due to MnDOT in April 2005.
Documents:
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